Don't need bricks & mortar sign to be a business.
I won't divulge anything specific here because my situation is tightly managed by my Accountant, but you need to get your own good Accountant, and the ability to Invoice your Employer rather than being an employee. Trading then gives you an additional income stream for example. Bring a partner into the business and it gets better...
Then - you get to decide what costs the company pays or reimburses you - for example, YOU own the car and you need to be reimbursed for travel. You run a small office, so anything associated with that are paid for in pre-tax dollars, even a percentage of the interest on your Mortgage is deductible - but be careful of CGT should you sell the family home. If renting, then less problems.
There is more paperwork and just one spreadsheet. I'd spend about an hour a week and then one more hour per quarter dealing with BAS, STP and SGC obligations.
It can be done.
TC.